To develop and apply techniques for the isolation of immunocompetent thymus-derived lymphocytes. To characterize in a quantitative and qualitative fashion the response of such cells to a variety of in vitro correlates of cellular immunity such as antigen-dependent lymphocyte proliferation and macrophage inhibitory factor production. To characterize the ultrastructure and physiology of these cells as contrasted with other types of lymphocytes. To study the mechanism of lymphocyte recognition of and activation by antigen and mitogen using visible antigenic markers such as horseradish peroxidase (HRPO), HRPO-protein conjugates and radiolabelled antigens. To characterize the role of macrophage-lymphoctye interaction in the regulation of antigen recognition by thymus-derived lymphocytes. To study the cellular and molecular basis of the genetic of immune responsiveness and the role of surface membrane determinants in cell cooperation.